Seat base



FIG. 1 is a perspective view of a seat base embodying my new design.

FIG. 2 is a front view of the design.

FIG. 3 is a side view of the design.

FIG. 4 is a back view of the design; and,

FIG. 5 is a top view of the design.

The broken line structure of the figures is for environmental purposesand forms no part of the claimed design.

The ornamental design for a seat base, as shown and described.